Modulating system



April 16, 1935. 5 BLACK I 1,998,115 4 MODULATING SYSTEM Filed June 27, 1934 IN l/E N TOR H 5. BLACK "ZMMM' ATTORNEY Patented Apr. 16, 1935 I umrso STATES PATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application June 27, 1934, Serial No. 732,591

3 Claims.

This invention relates to modulators, demodulators and detectors and more particularly to modulating systems in which a plurality of undesired currents are suppressed in the load circuit.

The main object of the invention is to improve the economy and reliability of modulating systems.

Another object is to increase the degree of suppression of the carrier and of unwanted currents in various portions of the system.

U. S. Patent 1,855,576, issued April 26, 1932, to C. E. Keith shows a modulating system providing for the suppression of harmonics in addition to the carrier and signal, thereby transmitting substantially sidebands alone. Other features therein include the provision of three mutually conjugate circuit branches and the adaptability of the system for simultaneous use as a modulator and a demodulator. Keiths arrangement utilizes balanced transformers, eight critically adjusted windings in all being required, together with four rectifiers which serve as modulating elements. In the present state of the art, transformers are expensive, particularly if the windings are balanced. On the other hand, rectifiers of the copper oxide type, for example, are cheap and efiicient.

In accordance with the present invention, two bridge type modulators having four rectifiers apiece are combined with two transformers to secure the desired suppression. The number of critically adjusted windings is thereby reduced to four which more than offsets the cost of the additional rectifiers employed. The invention possesses all the above mentioned features of the Keith circuit in addition to greater simplicity and lower cost.

The invention will be more readily understood by reference to the drawing which discloses one embodiment of the invention in which three circuit branches are each balanced with respect to the other two by means of bridges composed of rectifier units. Because of the balanced relation of the component circuits a signal source may be applied to one branch, a source of carrier waves to another branch a modulated current composed principally of the sidebands may be withdrawn from the third branch. The system comprises transformers !9 and 2t together with rectifier bridges 2i and 22. In the drawing the signal source is shown as a microphone l which is connected in series with a battery and winding 2 of transformer is; the source of carrier waves is shown as a carrier frequency generator 5, connected between the common terminal l8 of windings 3 and 4 of transformer 59 and the junction point 23 of the two rectifier bridges; and the load is connected to the transformer 28 comprising windings i4, i5 and it. The two bridge networks comprise four rectifiers each, numbered from S to E3, inclusive. Any rectifying means may be used which has a higher resistance to currents in one direction than in the other, such as a copper oxide rectifier.

The conjugate relations of the circuit branches and the action of the network in producing modulated currents in transformer winding I6 when signal currents are impressed upon transformer winding 2 and carrier waves are impressed between .the common terminal N3 of transformer windings-3 and 4 and the junction point 23 of the rectifier bridges will now be described.

Signal currents generated in transmitter l and flowing through transformer winding 2 will cause currents to flow in windings 3 and 4. Let it be assumed that the instantaneous value of this current is suchas to make the upper terminal of winding 3 of negative potential with respect to the mid-point l8 and the lower terminal of winding 4 of positive potential with respect to mid-point H3. The potential difference between the terminals of winding 4 will cause current to flow from the lower terminal of winding 4 through rectifier l3, through transformer winding I5 in a direction from its inner towards its outer terminal, through rectifier element it, and through carrier current generator 5 from right to left to the other terminal 18 of winding 4. Similarly, current will flow from the lower terminal 18 of winding 3, through carrier current generator 5 from left to right, rectifier element 9, transformer winding I4 in a direction from its inner terminal to its outer terminal, and. rectifier element 6 to the upper terminal of transformer winding 3. Provided the transformer windings l4 and I 5 are balanced and the currents are equal and opposite in direction as will be the case if the circuit is symmetrically constructed, no current will fiow in transformer winding I6 and consequently in load ll, under this condition. Also, the currents described as flowing through generator 5 willbe equal and opposite, resulting in no observable current through the generator. Also, if the instantaneous value of current is such as to make the upper terminal of winding 3 of positive potential with respect to mid-point l8 and the lower terminal of winding 4 of negative potential with respect to mid-point I8, the currents in windings I l and 15 will still be equal and in opposite direction. Thus a positive potential at the upper terminal of winding 3 will cause current to flow through rectifier 1, through transformer winding I4 in a direction from its inner to its outer terminal, through rectifier element 8, and generator 5 to the other terminal l8 of transformer winding 3. Similarly current will fiow from terminal l8 of transformer winding :3, through carrier current generator 5, rectifier element ll, transformer winding 15 in a direction from its inner to its outer terminals, through rectifier I2 to the lower terminal of transformer winding 4. Since these currents also traverse windings I4 and I5 in opposite directions and are equal in intensity, it is clear that they do not cause currents to flow in transformer I6 and load II. The currents again neutralize each other in generator 5. Thus neither portion of signal wave is transmitted either to the load I! or to the source 5.

In similar fashion, carrier currents from carrier current generator 5 are substantially balanced in transformer windings I4 and I5 and do not produce any current flow in transformer winding I 6. This will be seen if it is first assumed that the instantaneous value of carrier current is such as to make the potential of positive value at the common terminal I8 of transformer windings 3 and 4 and of negative value at the point 23,. In this case, current will flow from carrier current generator 5 through transformer winding 3 and in a direction from the mid-point I8 to the outer terminal, through rectifier element I, through transformer winding I4 in a direction from its inner to its outer terminal, and through rectifier element 8 to alternating current generator 5. Current also flows from carrier current generator 5 to the mid-point I8 of windings 3 and 4 through transformer winding 4 to its outer terminal, through rectifier element I 3, transformer winding I5 in a direction from its inner to its outer terminals, and rectifier element Ill to carrier current generator 5. The two currents produce equal and opposite magnetic effects in the windings 3 and 4, and again in the windings I and I 5 and thus have no eifect upon the windings 2 and I6.

If the instantaneous value of carrier current is such as to make the potential of negative value at the common terminal I8 of transformer windings 3 and 4 and of positive value at the junction 23, current will flow from generator 5 through rectifier II, through transformer winding I5 in a direction from its inner to its outer terminals, through rectifier I2, through transformer winding 4 in a direction from its lower terminal to the common terminal I8 and thence to carrier current generator 5. Similarly current will flow from carrier current generator 5 to the point 23, though rectifier 9, through transformer winding I 4 in a direction from its inner to its outer terminals, through rectifier element 6, through transformer winding 3 in a direction from its upper terminal to terminal I8 and thence to carrier current generator 5. Thus the impressed carrier has no effect upon the windings 2 and I5.

Inasmuch as signal and carrier waves are impressed jointly upon all the rectifiers, modulated waves arise in each rectifier and are transmitted to the transformer windings I4 and I5. These Waves are combined in additive phase relation in the winding I6 and delivered to the load II. In addition to the desired sidebands, various other modulation products are generated, some of which if not neutralized tend to distort the envelope of the modulated wave in the load. I find that those distortion products that are usually most prominent appear in phase opposition in the windings I4 and i5 and are neutralized. In particular, I find that harmonics of the signal and of the carrier are suppressed.

t is to be noted that the load I1 is conjugate to the signal source I and the carrier source 5, from which condition it is evident that either source may be made to change places with the load without disturbing the operation of the system. The conjugacy may be further demonstrated by supposing a voltage impressed across the winding I6 in such direction as to induce a current in the winding I4 from the inner to the outer terminal. The current will flow thence through the bridge 2| back to the inner terminal of the winding I4. The bridge being balanced, no current flows to either of the sources I and 5. The assumed voltage in the winding I6 will also induce a voltage in the winding I5 but no material current will flow because the rectifiers Ill, II, I2 and I 3 are all opposed to-the induced voltage and, in addition, the bridge 22 is balanced.

Assuming next that the voltage impressed across the winding I6 is reversed, the balanced bridge ill and its component rectifiers oppose the voltage induced in the winding I4. The winding I5 induces a current in the bridge 22, which is conductive but balanced with respect to the sources I and 5. Thus it is immaterial in what manner the sources and the load are transposed, for the state of conjugacy among the circuit branches is unaffected thereby. Also the load circuit may be employed to impress an incoming modulated wave upon the rectifiers together with the carrier wave from the source 5 to reproduce signals in any receiving device located in the circuit of the transmitter I.

What is claimed is:

1. A modulating system comprising two rectifier bridges, a signal source, a carrier source, and a load circuit, means to connect one source to both bridges in like phase, means to connect the other source to both bridges in opposite phase, and means to connect the load circuit to each bridge in a branch thereof that is conjugate to both sources, whereby harmonics are suppressed in the load circuit in addition to the carrier and signal waves.

2. A modulating system comprising two rectifier bridges, input and output transformers having three windings each, one winding of the input transformer being connected to each of the respective bridges, and one winding of the output transformer being connected in conjugate relation to the respective input transformer winding in each bridge, and a pair of terminals having connections to the two above mentioned input transformer windings in parallel, said connections and the third winding of each transformer constituting three mutually conjugate circuit branches.

3. In a modulator, two rectifier bridges connected together at a common corner, an input transformer having a divided secondary winding, a carrier source connected between the common bridge corner and the division point of said secondary, connections from the extremities of said winding to the respective bridge corners diagonally opposite the common corner, a signal source across the primary winding, and an output transformer with three windings, two of which are connected respectively across the remaining bridge diagonals, the third winding being connected to a load circuit.

HAROLD S. BLACK. 

